As you purchase the file, you need to download and install the zip file that you received in the email.
Now in your purchase receipt, you will see the license key on your screen, copy the key and paste it into the WPSchoolPress Settings’ License Tab.

OR

You can login to WPSchoolPress website and get your purchase details like purchase history, downloads, license keys
Copy the license key and add it in your WPSchoolPress General Settings